In an internal combustion engine, it is generally useful to measure an air-fuel ratio of a combustion process to efficiently control the engine's performance. In particular, by controlling the air-fuel ratio such that the mixture for combustion is stoichiometric, exhaust emissions are reduced by enabling a downstream catalytic converter to operate with maximum conversion efficiency for the principal exhaust constituents.
Various schemes have been used to measure the electromagnetic energy, in the form of light, emitted during the combustion process for indirectly determining or controlling air-fuel ratio. These previous schemes have not been sufficiently simple in their implementation. For instance, one approach suggests determining air-fuel ratio by considering the phase difference between peak values of the outputs of two photoelectric sensors tuned to receive substantially different wavelengths. Thus both the peak values of light intensity and the phasing at which they occur must be detected. This adds additional complexity and inaccuracy. The final result is a non-linear relationship between the measured parameter and the air-fuel ratio. To convert this non-linear result into usable form, requires additional resources. This also adds inaccuracy to the determination process. Another approach uses the outputs of two photoelectric sensors whose peak responses are at different frequencies. The ratio of the logarithm of the output of these sensors is considered at a specific point in the combustion process. The magnitude of the minimum of the signal after the peak value of the signal is claimed to be related to the NOx content of exhaust gas. In this approach, the air-fuel ratio is not directly derived, but inferred from the resulting signal. This approach is both complex and indirect.
Other schemes that directly measure intensity of emitted electromagnetic radiation are inaccurate because of the contamination of the measuring device, mainly due to soot deposits that vary according to engine operating conditions.
What is needed is a more robust and accurate apparatus, or method that is simpler to implement, for directly measuring the air-fuel ratio of a combustion process in an internal combustion engine.